Find and Fix a Database Performance Issue in Ruby on Rails

• ruby

In this chat with AppMap Navie, the user received an alert from their APM on poorly performing code. Using AppMap Navie they can ask it to give detailed advice about how to improve the performance of this API endpoint.

👤 USER

Tell me how to fix the issue that is causing current_user to be called repeatedly on my users listing page.

NAVIE

To address the issue of current_user being repetitively called on the users listing page, which likely leads to unnecessary database queries and reduces the page's performance, we can consider caching the value of current_user. This could avoid redundant database hits by fetching the user details from the cache if available, instead of querying the database each time the method is called.

From the provided sequence diagrams and code snippets, it's evident that the current_user method, defined in app/helpers/sessions_helper.rb, makes several conditional checks to determine the current user, possibly resulting in repeated database queries to fetch the user's information.

Optimizing current_user Method

The current rendition of the current_user method fetches the user record from the database on each call. We can modify this method to cache the user object for the duration of the request, reducing database queries. The ||= operator is commonly used in Ruby for memoization, storing the result of an expensive operation in a variable for reuse.

Implementation:

1. Refactor the current_user method to utilize memoization:

  
# app/helpers/sessions_helper.rb
def current_user
  # If @current_user has not been set, it evaluates the following; otherwise, returns @current_user
  @current_user ||= begin
    if user_id = session[:user_id]
      User.find_by(id: user_id)
    elsif user_id = cookies.encrypted[:user_id]
      user = User.find_by(id: user_id)
      user if user && user.authenticated?(:remember, cookies[:remember_token])
    end
  end
end
  

By using @current_user ||=, the method will only make a database call if @current_user is nil, significantly reducing the number of queries on pages where current_user is called repeatedly.
2. Review Other Parts of Your Application:
Ensure that other parts of your application, such as views or helpers, are not unnecessarily triggering database queries or heavy computations by relying on methods similar to current_user. For instance, conditionally displaying user-specific information in views might not require a database query every time if you efficiently cache and access user session data.
3. Database Indexes:
Make sure the users' table's id column, and any other columns used frequently in lookups (user_id, remember_token), are indexed. This will improve the speed of these queries, although it doesn't reduce their quantity.
4. Consider Middleware or Controller-Based Caching for more complex scenarios:
For applications with more complex requirements around user data, consider implementing caching strategies at the middleware or controller level. Rails' Low-Level Caching or Fragment Caching might be suitable for preserving portions of your page or specific objects.
Monitoring Changes:

After implementing these changes, monitor your application's performance using AppMap again to ensure the optimizations have the desired effect. Look out for a reduced number of current_user-related queries in the sequence diagrams and an overall improvement in page response times.

Remember, caching comes with its trade-offs, such as potentially serving stale data, so it's crucial to implement a strategy that invalidates the cache appropriately. For user authentication data, this typically involves setting reasonable expiration times or explicitly invalidating the cache when users log out or update their profile.

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